A light-emitting diode (LED) is a semiconductor device that emits incoherent narrow-spectrum light when electrically biased in the forward direction. This effect is a form of electroluminescence. The color of the emitted light depends on the composition and condition of the semi-conducting material used, and can be infrared, visible or near-ultraviolet. Like a normal diode, it consists of a chip of semi-conducting material impregnated, or doped, with impurities to create a p-n junction, as current flows easily from the p-side, or anode, to the n-side, or cathode, but not in the reverse direction. Charge-carriers, which are electrons and electron holes, flow into the junction from electrodes with different voltages. When an electron meets a hole, it falls into a lower energy level, and releases energy in the form of a photon.
LEDs were first developed with infrared and red devices made with gallium arsenide. However, advances in materials science have made possible the production of devices with ever-shorter wavelengths, producing light in a variety of colors. LEDs are conventionally built on an n-type substrate, such as sapphire, with electrode attached to the p-type layer deposited on its surface. While the less common P-type substrates may occur as well. Substrates that are transparent to the emitted wavelength, and backed by a reflective layer, increase the LED efficiency. The refractive index of the package material should match the index of the semiconductor. A disadvantage with the LED is that the light produced gets partially reflected back into the semiconductor, where it gets absorbed and turns into additional heat.
An embodiment of a white LED is disclosed in United States Published Patent Application Number 20070024191. The white light emitting diode (LED) includes a reflective mirror arranged on the light emitting path of a blue or an ultra violet LED die at an appropriate angle. Phosphors are coated on the reflective mirror, the emitting plane of the LED, or both so that the phosphors are excited by the blue or UV lights emitted by the LED die to produce white lights. The white LED has a long lifetime and a uniform light color by separating the phosphors from the LED die, and by allowing the lights emitted from the LED die to undergo several excitations with the phosphors.
Typically, in applications where light from a single LED is insufficient, several LEDs need to be put together to provide illumination. This is for example, the case in a backlight unit, such as display which relies on a number of white LEDs to provide illumination to the display. It should be obvious to a person skilled in the art that LEDs are advantageously used in all kinds of devices for performing different jobs. Among other things, they form the numbers on digital clocks, transmit information from remote controls, light up watches and indicate when your appliances are turned in use. Collected together, they can form images on displays, such as laptop computers, personal digital assistants, mobile phones, television screen or even illuminate a traffic light. It is therefore preferable that such devices and/or displays exhibit a uniform color temperature. However, in certain types of LEDs coated with a phosphor to generate white light, the so called white LEDs, exhibit variations in color temperature due to differences in raw material sources, crystal growth, handling, storage conditions for the raw materials, and the other variables that go into the manufacturing process. Therefore, the LEDs that are manufactured required to be binned before being assembled into the backlight unit of a display. The binning methods typically use color coordinates or correlated color temperature (CCT) for sorting the LEDs, but LEDs with the same CCT can still have a different color tint. Manufacturers have used new binning strategies that tend to reduce the variability of LEDs within each bin. Such binning processes significantly increase the cost of manufacturing the LEDs.
Therefore a need exists to provide an improved LED capable of providing white light ameliorating one or more of the above mentioned disadvantages. Without a way to provide the improved LED capable of generating white light of a desired color temperature and the method of generating white light from of a desired color point using the improved LED, the promise of this technology may never be fully achieved.